This application pertains to the same general subject matter as co-pending applications, Ser. No. 585,861 filed Mar. 2, 1984 now U.S. Pat. No. 4,543,176, by Hara et al, and Ser. No. 623,219 filed June 21, 1984 now U.S. Pat. No. 4,548,179 by Ninomiya et al but the specific subject matter taught and and claimed is different therefrom.
This invention relates generally to an air/fuel ratio contol system for an internal combustion engine used for motor vehicles or the like, and particularyly, to apparatus for heating an oxygen sensor used for detecting the concentration of oxygen in exhaust gases.
In a typical air/fuel ratio control system used for internal combustion engines of motor vehicles of the type using a three-way catalytic converter, an oxygen sensor is used to detect the concentration of residual oxygen included in exhausted gases from the engine so as to control the air/fuel ratio to be close to a stoichiometric value. Such oxygen sensors have a given operating temperature range in which a detection current corresponding to the concentration of oxygen can be obtained, and in order to keep the temperature of such a sensor at a high value, such as above 500.degree. C., a plutinum heater is built therein to continuously heat the body of the sensor during operation.
However, since the heater is continuously energized to heat the sensor, in the case that the sensor and the heater buit therein are overheated to a temperture such as above 1,400.degree. C. due to abnormal overheating of the exhaust gases, breaking of the heater wire is apt to occur to stop normal detecting operation. Abnormal state of the oxygen sensor may cause the air/fuel ratio control system to supply the engine with an excessively lean mixture. To prevent such undesirable phenemenon therefore, feedback control using the oxygen sensor output is terminated on such abnormal condition and the air/fuel ratio is controlled to a setting value by way of an open loop control as described in Japanese Patent Provisional Publication No. 57-140539. Although this technique of switching from feedback control to open loop control avoids extremely undesired control of an air/fuel mixture, it does not provide a basic solution since the feedback control cannot be recovered unless the broken sensor is replaced with a normal one or repaired.